Vacuum tube-transistor coupling circuit coupling drive power from vacuum tube to power transistor to load



Nov. 24, 1964 R. s. WEBB 3,153,7

VACUUM TUBE-TRANSISTOR COUPLING CIRCUIT COUPLING DRIVE POWER FROM VACUUM TUBE TO POWER TRANSISTOR TO LOAD Filed Dec. 18, 1961 2 Sheets-Sheet 1 Nov. 24, 1964 R. s. WEBB 3,

VACUUM TUBE-TRANSISTOR COUPLING CIRCUIT COUPLING DRIVE POWER FROM VACUUM TUBE TO POWER TRANSISTOR TO LOAD Filed Dec. 18, 1961 2 Sheets-Sheet 2 A24 INVENTOR. 30%:2? 3 WeJ United States Patent AQUUF/Z TEL .4 F LlN EST J QUE? QQLTLn G D 31 E P9? ghway, Troy, h ch. Eel, No. 163,156 ms, {CLSQL-Sd) This invention relates to an interstage coup 'ng circuit for an electronic amplifier, particularly to a network for coupling a stage of vacuum tube amp ification to a common emitter transistor network.

'Ihere are a number of applications for high power electronic pulse circuitry having adjustable On-Gil ratio at a particular frequency. One application for such circuitry is in an electrical discharge machining power circuit.

Electrical discharge machining, sometimes referred to in the art as EDM, spark machining, or are machining, is carried on by passing a series of discrete, localized, extremely lugh current density discharges across a gap between a tool electrode and a conductive workpiece at sonic or ultrasonic frequencies in the presence of a dielectric luid for eroding the workpiece.

in electrical discharge machining the conductive tool is usually maintained in proximate position with the workpiece by an automatic servo feed and is advanced toward or into "the workpiece as stock is removed therefrom.

A fluid coolant, usually a liquid, is circulate the working gap to flush the eroded particles from the gap and is sometimes furnished under pressure by a pump through a pattern of holes in th electrode. The defining characteristic of electrical discharge machining is that the coolant is a dielectric such as kerosene, transformer oil or pure water andis broken down i localized areas by the action of the machining power supply between the closest points of the tool and work.

An example of the type of EDM power circuitry representative of the present state of the art is shown in Matulaitis and Lobur Patent No. 2,951,969, issued September 6, l960. A machining power circuit, such as discolsed therein, when combined with a power feed of an improved type as shown in my Fatent No. 2,962,630, issued November 29, 1960, results in a machine having excellent control characteristics and readily usable by machinists having mechanical Slilll only @d no electrical or electronics background as is typical of the machining trades.

The above mentioned machin-ng power circuit contemplates a fixed multivibrator cycle for producing a delinlte on-tirne and oil-time at a particular machining tap. I have found that it is desirable to be able to vary continuously the machine on-time wh le maintaining a substantially constant frequency or repetition rate. This variation in on-time or duty factor can range from as little as one percent or less to eighty percent or more. A machine havin these characteristics has infinitesimally adjustable machining current or machining rate thereby permitting exact adjustment to maximum er'liciency machining conditions at a particular desired metal removal rate and surface finish or machining gap.

A further objection to the circuitry of Patent No. 2,951,969 is that a machining power circuit capable of extremely high current cut ut required for high metal removal rate necessitates the use of many, someti es thousands of vacuum tubes to produce the desired niachining current. Since vacuum tubes are inherently high voltage devices, extremely high power losses result at these machining currents and expensive equipment and through ice compl x electronic circuitry is required. The cost of operation and consumption of electric power is correspond ingly high. Furthermore, vacuum tubes are thermionic emissive devices and their average or rated life is approximately 10% hours.

hat as the number of vacuum tubes increases, the cost of he basic mach e, as well as operation and maintenance becomes excessive.

Accordingly, it is the principal object of this invention to provide an lproved machining power circuit, having widely adjustable oil-off ratio or duty factor infinitesimally adjustable throughout the machining current range at various selected discharge repetition rates; having transistors as the electronic swi h for controlling the discharge characteristics and employin an emcient coupling network for coupling a vacuum tube amplifier to a transistor amplifier.

Another object of this invention is to provide means for positively driving t e transistor bank with accelerated turn On and turn Off ch acteristics under power.

Another object of this invention is to provide a shuntchoke network for sharply accelerating turn Oil of a transistor bank driven by a wide range pulse circuit.

Still another object of this invention is to provide a a simple and reliable means for providing a self developi -g low voltage-high current bias for maintaining the transistor bank off during periods of nonconduction.

{Ether obg ects and advantages are disclosed in the follov g specification which taken in conjuctionv/ith the accompanying drawings show preferred forms of apparatus for practicing the invention.

In the drawings in which reference numerals have been used to designate like parts herein referred to:

Fl-G. 1 shows in schematic form a pulse generating and amplifying c rcuit having a continuously adjustable on-ofi ratio at several different pulse repetition rates and the associated pulse transformer circuit for coupling the output of the vacuum tube amplifier to drive the transistor bank;

HG. 2 shows a voltage waveform across the pulse transformer for the circuit of PEG. 1 during a relatively narrow on-time;

3. 3 shows a similar voltage waveform during a conn of relatively wide on-ti. -e; P16. 4- shows a modification of the FIG. 1 circuit includ n g a shunt-choke network for turn ofi of the transistor bank;

5 shows a voltage waveform across the pulse transformer for the circuit of REG. 4 during a condition of narrow on-tirne;

ZlG. 6 shows a corresponding voltage waveform across the pulse transformer of FIG. 4 during a condition of wide on-time.

Referring now to PEG. 1, the pulsating drive signal is generated by multivibrator tubes l d and 12 connected respecti ely through plate load resistors 14- and in to the positive terminal of anode supply 18. The negative terminal of DC. supply 13 is in turn connected to the positive terminal of supply Power su lies 1% and 2% together comprise the anode power supply for the multivibrator and first stage of amplification. A tap 22 connects between supplies 1; and and provides screen voltage for the various stages of pentocle amplification. screen grid of tube i connects through a resistor 24 to the common screen tap of the multivibrator circuit. The screen grid of tube 12 connects through a resistor 26 to the common tap which is in turn connected through a resistor 26 to scr en tap 22 of the main supply. A bypass capacitor 3 connects the common junction of resistors 25 and 23 to the common ground of the system which is represented by a lead 32.

Typical cross-coupling capacitors 3d and 36 are selected With this limited life, it can be seen of tube 56 from the multivibrator circuit.

. 3 V by frequency selector switches 38 and40 and cross-couple the anodes of tubes 19 and 12 to the opposing grid.

' The control grid of tube 12 is returned to the positive terin the string is chosen to determine the minimum desired bias voltage across capacitor 112.

of reference diode 52 and capacitor 54'is connected to the junction between resistors 58 and 69 as shown. Resistor 58 returns the grid of tube 56 to negative bias 62 and during periodsof off-time for tube 56 maintains this tube nonconduct-ive. Resistor 60 limits'grid current during 'per-iodsof on-tirne thus decoupling thecontrol grid A small capacitor 64 is connected across resistor 60 to provide sharp leading edge drive to tube 56 thereby insuring sharp switching characteristics of this tube. V

The output of tube 56 is developed across a plate load resistor 65 which returns the anode of tube 56 to the anode power supply 18.

A coupling capacitor 66 couples the signal output of tube 56 to the grid of drive tube 68. The grid of tube 'patents. 7

terminal of a machining power supply 132. The posi The emitter of transistor 110 is connected through a balancing resistor 122 to reference ground lead 32. The collector of transistor 110 is connected through a power resistor bank 124..having a capacitor 126 connected in parallel therewith. The opposite end of this network is connected to a work-piece 128 in this example. Electrode 130 is maintained in spaced proximity to the workpiece as described in detail in the above mentioned The electrode 138 is connected to the negative tive terminal of machining power supply 132 is'con- ,Jnected 'to reference ground 32 thereby completing the 68 is clamped to bias 62 through a diode 70. A resistor 72 is connected in parallel with diode 70 to provide off bias for tube 68 during periods of its nonconduction. A resistor 74 limits the grid current of tube 68 in a manner similar to resistor 60 of tube 56.

The control grid of a tube 76 is returned by a lead 78 to the negative terminal of reference diode 52. A

resistor 80 limits grid current from tube 76 and a resistor 82 provides a localized bias return for this tube. Amplifier tubes 68 and 76 are generally comprised of a bank of several tubes for providing sufiicient drive power to, successive stages. a

The anode of tube 68 is connected through a lead 84 to a network comprising resistor 86 and shunt connected capacitor 88; p The opposite side of this network is connected .to one side of a primary 90 of drive coupling transformer 92. A lead 94 provides a center tap connection for the primary of this transformer and con- 7 meets the 'anode circuit of tube 68 to the positive terminal of a drive power supply 96.

- Tube 76 is connected to alead 98 to a resistor 100 supply.

The secondary of transformer 92 is connected to drive a transistor bank 119 alternately conductive and nonconductive. One side of the secondary of transformer 92 is connected to the positive terminal of a network consisting of'capacitor 112and shunt connected diodes 114. The negative terminal of this network is connected to reference ground 32. The opposite side of'the secondary of transformer 92 is connected through a diode 116 to the base of transistor 110. Connected in parallel with diode 116 is a resistor 118 and a capacitor 120.

In this example, transistor 110 is a PNP device and the anode of diode 116 is connected to the base of transistor 110. Diodes 114 are'silicon or germanium devices and have a minimum conduction voltage of approx-imately e to .8 volt each. The cathode of this diode string is connected to the negative terminal of capacitor 112.. and the number of series connected diodes s'pective grid circuits.

discharge loop.

Operation of thiscircuit is controlled by multivibrator tubes 10 and 12. The novel grid return of thismultivibrator is extremely important :in a modern electrical discharge machining power circuit because of the wide range of on-ofi ratio or duty factor required at a particular frequency. This is achieved by returning the If coupling capacitors 34 and 36 are equal, the formula may be simplified to:

From this simplified formula, it may be seen that as the tap on rheostat 46 is moved from one extreme to the other, resistance is similarly moved from one grid return to the opposing return; thereby maintaining a constant frequency regardless of the position of the adjustable tap on rheostat 46, thus achieving a Wide range in on-off ratio at a fixed frequency.

Except for this important grid return, the operation of multivibrator tubes 10 and 12 is well known in the electronics industry for given coupling capacitors 34 and 36. To select a different operating frequency, switches 38 and 40 which are normally ganged together as a frequency control, select dilferent capacitors in the re- In this example, six frequency taps are shown. The output of multivibrator tube 12 is, produced' across plate load resistor 16 and is a substantially rectangular signal having a frequency determined by the composite grid circuit resistance in conjunction with capactors 34 and 36 and a duty factor determined by the setting of the adjustable tap on rheostat 46.

Another important circuit network required to achieve this widely adjustable duty factor is the network for coupling the output of the signal from the multivibrator to successive stages of the amplifier. Analysis of this circuit will shown that multivibrator tube 12 and power transistor bank are On or conductive in-phase with a discharge across the gap between electrode and 1 referencing a rectangular or pulse output developed across signal resistor 16 downward as is required for proper control of the grid of amplifier tube 56. Bias return resistor 58 maintains tube 56 nonconductive during periods of nonconduction of tube 56. This is achieved as multivibrator tube 12 becomes conductive, thereby generating a voltage drop across resistor 16. A typical value for plate supply voltage it? and 213 is approximately 250 volts total and the drop across rnultivibrator tube 12 during conduction is approximately lOQ volts, therefore generating a signal of approximately 150 volts across resistor 16.

As long as the regulated voltage of reference diode 52 is larger than the drop across tube 12, the control grid of tube So will at this time be negative. In the example given, the reference diode would have a magnitude of regulation of approximately 200 volts and the control grid of tube 56 would therefore be biased to ap oximately minus 100 volt Reference diode 52 and capacitor 54 form a network to: producing a floating 11C. voltage having almost no capacitive losses during switching as would be encountered if a D.C. supply of normal characteristics produced from a transformer winding, rectitiers, etc, were employed at this point. The sure. or shunt or leakage capacity re ulting from this improved network which is fioatinc on anode of tube 12 to lead 32 or other circuit components, results in far less shunt losses than would occur with a conventional 11C. supply connected at this point.

As rnultivibrator tube 32 becomes nonconductive, the voltage signal across resistor 16 disa" ears, thereby carrying the reference d' de circuit positive. In this example, with a 256 volt a 280 volt reference diode, approximately 50 volts is do loped across resistor 6% connected to the control grid of tube 5-5 since the control grid clips the signal at approximately the some voltage as the cathode of tube An extremely small lead capacitor is connected across resistor 6%} for improving the sharp rise and fall charactistics of this signal, thereby causing arnpL'fier 56 to both amplify and resquare the signal as well as decouple the multivibl'ator from successive stages of amplification.

Rectangular pulsating signal or" amplified power is therefore generated across load resistor 65 connected in the anode circuit of tube 56 and is coupled through capacitor 2'6 to the control grid circuit of tube Diode 7 clamps this pulsating signal to bias 552 and resistor '74 limits grid current during periods of conduction of tube 63 since the signal developed across resistor 72 is in excess of the bias voltage 52.

Gener ly the off-time or duration between pulses of the output transistor bani; is at least twenty percent of the total signal. In other words, the duty tor is approximat ly eighty percent compared to a 1- rnum duty factor of Ainiately one percent. Since tube 56 is therefore conductive for at least twenty percent of the cycle, sufficient power is transferred through coupling capacitor to cve the control grid tube 33 and a norrnal coupling circuit may be employed in the control id ci cuit or" tube 6 The cont ol g d circuit of tube 76 is driven by the some source or" ial as tube 55 and is therefore connectcd to the re "1C6 diode network 52, 54 by lead 73. The drive cnaract stics of this tube are q e similar to that of tube '6 except that a number or" tubes may be employed in parallel in place of tube 76.

Power tu e r58 is driven On or conductive inphase with conduction of power transistor 1142. Electron fiow during rive of transistor lid is from lead 32 t rough tube as to anode lead During the instant or turn On, electron flow passes capacitor 8-5 since there is zero voltage across capacitor at the instant of turn On. Primary 9t; of transformer 92 completes the c writ to the positive terminal of drive power supply 96 thus completing the power circuit of tube 68.

Electron flow in primary of transformer 92. pro- Drive current causes transistor 11% to become cond ctive. i the gap between worl' iece and electrode tly small to perm a discharge, electron tlow occurs from the nega ve terminal of discharge power supply 132, electrode 139, workpiece 123, resistor 4.1 collector-emi ter of transistor 11%, balancing resistor 212 to the po e terminal of supply Capacitor is provided in shunt with resistor 124- to provide accelerated turn 911 of the discharge gap. The charging tir e of sis capa itor is selected to be small with respect to total period of conduction of transistor lit; and is permitted to discharge substantially between pulses.

After a per'od of conduction of transistor ill the transistor bank is driven off under power by conduction of tube bank '70". T urn Off electron tiow occurs in the primary circuit of transformer from lead 32, tube 75, lead 58, resistor 13%, primary 1:32, lead 94- to the positive terininal of dri e power upply 9d. Conduction of electron fiow in this winding is or" opposite phasand causes a reverse current flow in the secondary of transformer 92. During the period of turn Off, electron r'low occurs from the secondary of transformer 52 through capacitor istor I122, emitter-base of transistor capacitor 12%, to the opposite side of the secondary of transformer 2. After transistor ill) becomes nonconductive, the emit r-base junction blocks in this polarity and rot (is further current ii-ow. If this transistor is an alloy junction device or other type having a high reverse voltage rating between emitter and base, electron iiow does not continue except for the nainute amounts resulting from reverse leakage of the junction. Typical transistors employed in this bank, however, are the diffused junction type and are of such constmction that a relati' y low voltage in reverse across the ba e-ernitter junction causes reverse conduction. The magnitude of such reverse conduction is limited by resistor 113 since electron flow is blocked by diode 116 and is limited to a value that is safe or non-damaging t0 transistor lid and nonsaturating to transrorrner 92.

Regardless of transistor type, this drive circuit, particularly drive transformer 92 and the current limiting networks, is subject to particular consideration. As menthe duty factor at a particular frequency and in a ty ical advanced machining t will vary from a minimum of approximately 1 percent duty facor at a particular frequency to a maximorn of appro. r lately 8G or 96 percent duty factor or conduction period of the machining cycle. A transformer, on the other hand, is an A.C. device in which the positive voltage area oi either secondary or primary must exactly equal the negative voltage area. It is therefore virtually impossible and certainly impractical to operate transformer 2 2 in push-pull without saturating its iron core during periods of narrow ontime regardless of the type 01. transistor utilized; and proper circuit design is required to prevent saturation during periods of wide ontirne when the transformer is to rive a diilused junction transis or having extremely low emitter-base reverse voltage lirn t.

Operation of his circuit during these extreme conditions is best shown by comparing FIG-S, 2 and 3. Fi 2 shows a condition or" narrow on-tinie and FIG. 3 shows a condition of relatively wide on-tinie. The forward or drive voltage resulting from conduction of tube 63 is shown in P16. 2 as rising along line 134. it the secondary of transformer @2 were open circuited, the voltage would rise to magnitude 136 and the turns ratio is designed to permit this maximum voltage condition for ac celerated turn On of the transistor by breakdown of the uncoupled circuit inductance which inherently results from all lead lengths in the primary and secondary, the uncoupled inductance of the transformer and other unavoidable series inductance in the components themselves. As conduction occurs through the series inductance and through the transistor, the resulting maximum voltage across the transformer is shown as magnitude 138. After the delay. time of this series inductance and the breakdown of the transistor into a conducting condition, the voltage falls rapidly to 140 and thereafter remains constant during the period of conduction. This magnitude of conduction voltage is the total of the series drop due to circuit resistance, diode 116, the base-emitter junction of transistor 110 and the voltage across capacitor 112.

At the instant of turn Off, tube 68 is rendered nonconductive and tube 76 is rendered conductive, causing the voltage across transformer 92 to reverse. There are normally. fewer tubes in bank 76 than 68 and more turns on winding 102 than winding 90 such that the maximum reverse'voltage represented by imaginary line 142 is lower in magnitude than forward voltage 136. Reverse voltage is developed under power to magnitude 142 and upon blocking of the emitter-base junction of transistor 110,

' an oscillation occurs about this voltage magnitude caused by ringing of the stray capacity of this circuit and of the transistor junction itself. This oscillation shown at 144 does not affect operation of the circuit, however, since the transistor is biased OE during this period.

The positive area under the curve corresponding to a condition of forward drive is represented by area 146 and results from conduction of tube 68. The negative area of the transformer voltage waveform curve is shown as area 148 and occurs during conduction of tube 76. Areas 146 and 148 are of course equal. Tube 76 conducts duringrthe entire negative portion of the waveform and the transformer saturates during this negative portion of the wave causing the voltage to fall along line 150. Saturating the transformer in this polarity is nondamaging since the total circuit current flow is limited by primary resistor 100 connected in series with tube 76. Conduction of tube 76 during the entire negative portion of the wave prevents any oscillation or runaway condition of the transformer voltage whichmight result in a positive voltage during the off-time. .As the secondary voltage falls along line 150, the bias stored across capacitor 112 maintains transistor 110 nonconductive for the balance of the period of nonconduction until the next forward voltage pulse represented by line 152 occurs. Thus the transistor is turned 7 Olf under power from tube bank 76 and immediately after turn Off is maintained nonconductive by the bias across capacitor 112.

FIG. 3 shows a condition of long on-time or high duty factor in which the voltage rises along line 154. Magnitude 156 and 158 are identical respectively to magnitudes 138 and 136 and after leading edge breakdown into conduction falls to point 160 which corresponds to voltage 140 of FIG. 2.. Upon turn Off of the forward drive pulse, caused by conduction of tube 68, the drive voltage is carried under power by tube 76 to point 162. An imaginary line is shown at 164 and corresponds to 1420f FIG. 2, representing the maximum transformer voltage caused by conduction of tube 76. The positive area of 166'is likewise equal to the negative area 168 and for this condition of long on-time, results in the induced negative voltage being higher in magnitude than the conduction level of tube 76 and the portion of the negative area below line 164 is caused by the normal flyback action of the transformer. Nevertheless, the transformer is carried under power to point 162 sharply turning Off the transistor bank. As outlined above, the base-emitter junction then blocks and places no load on the transformer at this time.

If transistor 110 is of the alloy junctiontype, diode 116 resistor 118 'andcapacitor'120may be omitted since the transistor junction would be capable of withstanding the higher voltage magnitude shown at 170. If transistor is of the diffused alloy'type-having a lower base to emitter rating, these components must be included to limit reverse current through the transistor during this off-time and to prevent saturation of the transformer during the forward or drive voltage condition. In this case, capacitor is chosen to permit suflicient reverse surge of power to completely turn Off the transistor under power and resistor 118 is chosen to have as high an ohmic resistance as possible while insuring positive off bias of the transistor bank. It is necessary to design the maximum voltage output of tube 76 across winding 102 to the maximum reverse voltage 170 if appreciable current flows through resistor 118 during off-time. This is required so that the forward voltage along line 160 does not saturate under wide on-time conditions thus resulting in loss of drive. During this condition of relatively long on-time, the bias stored across capacitor 112 is ineffective since the voltage across the transformer secondary is sufficient to maintain off bias of the transistor.

FIG. 4 shows a modification of the FIG. 1 circuitry having a somewhat difierent drive network. The dis: charge power loop and transistor 110 are identical to that shown in FIG. 1 and all components shown in the discharge loop including components 122 through 132 are the same as those shown in FIG. 1. The multivibrator and first stage of amplification are also the same as in FIG. 1 and would be connected at terminals 172, 174, 176 and 178, as indicated in FIGS. 1 and 4.

Vacuum tube drive power bank 180 is similar in function to tube bank 68 of FIG. 1 but consists of more tubes connectedin parallel. Diode 182 is similar in function to diode 70 and serves to clamp drive signal to bias 62 at point 174. Resistors 184 and 186 serve similar functions to resistors 72 and 74 of FIG. 1. The anode of tube 180 is connected through resistor 188 to one sideof the single primary of drive transformer 190. A lead capacitor 192 is connected in parallel with resistor 188. The opposite side of the primary winding of transformer is connected through lead 194 to the positive terminal of drive power supply 196..

One side of the secondary of transformer 190 is connected to reference ground 32. The opposite side of the secondary of transformer 190 is connected through diode 198 to the negative terminal of capacitor 200. A series diodenetwork 202 is connected across capacitor 200 with the cathode of the diode network on the negative terminal of the capacitor. The common positive junction of capacitor 200 and diode network 202 is connected to the base of transistor bank 110. One lead of the choke 204 is connected to the junction between capacitor 200 and diode 198. The opposite end of choke 204 is connected through a resistor 206 to reference ground 32, thus completing the drive loop for transistor 110.

Operation of the circuit of FIG. 4 is such that forward drive current to transistor 110 is caused by conduction of tube 180 in a manner similar to the circuitry of FIG. 1. Conduction of drive electron flow in the primary circuit occurs from the negative terminal of supply 196 through common ground 32, tube 180, network 188-192, primary of transformer 190, lead 194, to the positive terminal of supply 196. Capacitor 192 serves to accelerate conduction of current flow and turn On of transistor 110 by bypassing resistor 188 at the instant of turn On. Electron flow occurs in the secondary circuit from the secondary of transformer 190 through diode 198, capacitor 200, base-emitter of transistor 110, balancing resistor 122, reference ground 32, to the opposite side of the secondary of transformer 190.

After the initial delay period determined by the time constant of choke 204, a shunt electron flow occurs through choke 204 and resistor 206. Bias capacitor 200 is similar in function to capacitor 112 of FIG. .1 and FIG. 3 and is identical in every respect.

3 diode network 292 serves to limit the voltage across capacitor 2% just as diodes 114 limit the voltage across capacitor 112 in FIG. 1.

In the circuitry of FIG. 4, the shunt network consisting of choke 2G4 and resistor 2&6 serves to accelerate turn Oil of transistor 11%. As tube bank is rendered sharply nonconductive, the voltage across the secondary of transformer 19% tends to reverse. At the same instant, the electron flow through choke 2% is sustained by the induction of that choke and causes a turn electron flow through the transistor bank through resistor 2G5, resistor 3.22, emitter-base of transistor lid and bias capacitor 269. Thus transistor 11% is turn O'if under power by the inductive surge of inductance After the decay of current of inductance 2%, as determined by its time constant, transistor 11% is ained nonconductive by the bias stored across capacitor The flyback voltage of the secondary of transformer 19% is blocked from the circuit by rectifier 1% and discharge of capacitor 2% through the secondary winding is similarly blocked by diode 198; thus insuring pulse operation of transformer 1%.

Operation of the circuit of FIG. 4 is best understood by considering the voltage waveforms shown in F163. 5 and 6. These are voltage waveforms of transformer FIG. 5 corresponds to FIG. 2 for a condition of narrow on-time of transistor 116. FIG. 6 corresponds to PEG. 3 and shows a voltage waveform across transformer 11%) for a condition of relatively wide on-time or high duty factor.

The forward voltage waveform of FIG. 5 corresponds to the forward voltage waveform of FlG. 2 and is shown as waveform 265 in FIG. 5 and is identical in every respect to thatof FIG. 2. The reverse voltage produced across transformer 190 is somewhat ditierent than that or" transformer 52 since this results from the normal fiyback action of the transformer. The voltage is carried sharply negative along line 219 by the inductive action of choke 204 although no drive power is flowing into or out of the transformer winding at this time. Upon decay of the inductive effect of choke 2%, the voltage falls along line 212 and rings as shown at 21 This ringing results from the blocking characteristics and the minute capacitive leakage of diode 193 as well as the shunt capacity in the windings of transformer 199. Since this condition of the transformer fiyback is not driven under power by a. tube bank, no saturation results from this form of operation.

FIG. 6 shows a condition of wide on-time, as described above, and the positive waveform of FIG. 6, shown as waveform 216, corresponds to the same waveform of Upon turn St? of tube 183, the voltage across the transformer during this condition of wide on-time is carried negative by choke 204 and the turn 65 characteristic 218 is similar to 210 of FlG. 5. As in all other cases, the fiyback area must exactly equal the forward area and area 22% results in flyback voltage peak 222.

By comparing FIGS. 2 and 5 and FIGS. 3 and 6, respectively, it can be seen that there are only minor differences in the voltage waveform across the respective transformers, however, operation of the two circuits is considerably different. The important similarity in operation of the two circuits is that the transistor is sharply turned On under power and more important, it is turned Off under power and after the initial turn Oif period is maintained nonconductive by the low voltage bias stored across capacitors 12% or 200 in the respective circuits.

While the two circuits shown relate directly to electrical discharge machining, the important concept of each circuit is the novel transformer coupling netw rk for couplingthe pulsating output of a vacuum tube bank to drive a transistor bank. Other pulse circuits requiring efficient coupling of relatively large amounts of power it? from a vacuum tube pulse amplifier to a transistor power bank could utilize this same drive network, the only difference being that workpiece 121% and electrode comprising the working gap would be replaced by the new load circuit.

In the drawings, the DC. supplies are shown as batteries in the interest of simplifying the disclosure. In actual practice, these sources of DC. are derived from the secondary of a transformer having its primary con nected to the power source for the machine which may be single phase or polyphase AC. The secondary voltage is rectified and stored usually in an electrolytic storage capacitor to form a nearly idem DC. source having very low internal im edance.

It may thus be seen that I have shown and described a new and improved coupling network for coupling output of a vacuum tube amplifier to drive a transistm bank in which the coupling network is particularly adapt to pulse circuitry having a wide range of duty factor and pulse repetition rate. By disclosin the above circuitry embodying this invention, it is not intended to limit the scope of the invention to the above disclosure which is given for the purpose of example but only as set forth in the following claims.

1 claim:

1. A coupling circuit for connecting drive power from a periodically conductive, vacuum tube amplifier to at least one power transistor, having a base, emitter and collector, said transistor having a load and a power source connected in series across its emitter and collector and operable to receive input signals across its emitter and base cor prising, a pluse transformer having its primary operatively connected to the out at of said amplifier and its secondary connected across said base and emitter of said transistor, an inductance connected across said secondary for accelerating turn of said transistor, and a biasing means connected in series with said secondary for maintaining said transistor nonconductive for a selected time duration after turn OFF signal.

2. The combination as set forth in claim 1 in which said biasing means is self-developed by drive current and ompises at least one semiconductor rectifier connected in parallel with a capacitor.

3. The combination as set forth in claim 1 in which a parallel resistor-capacitor network is connected in series with said primary for accelerating turn ON of said transister.

4. The combination as set forth in claim 1 in which a rectifier is connected in series with said secondary for blocking llyback voltage.

5. A coupling circuit for connecting drive power from a periodically pulsed, push-pull, vacuum tube amplifier to at lea one power transistor having a base emitter and collec" 1', said transistor having a load and a power source serially connected across its emitter and collector and operable to receive output signal from said amplificr across its emitter and base comprising, a stepdown transformer having its primary connected to the output of said amplifier and its secondary connected across said base and emitter of said transistor, said amplifier oper able to pulse said primary with voltages of opposite pol 'ty to provide alternate turn ON and turn OFF signzus to said transistor, and a biasing means in series with said secondary for maintaining said transistor nonconductive for a selected time duration after its turn OFF.

6. The combination as set forth in claim 5 in which a parallel resistor-c nacitor network is connected in series w-.. said primary for accelerating turn ON of said transistor.

7. The combination as set forth in claim 5 in which said biasing means is self-developed by drive current and comprises a capacitor shunted by at least one semiconductor rectifier.

8. The combination as set forth n claim 5 in which transistor during OFF time andto 1 1 said vacuum tube amplifier includes apair of tubes, each havingits principal electrodes operatively connected to a difierent end of said primary, and a tap of said primary is connected to one terminal of a source of excitation voltage, one of said tubes operable to provide a turn ON signal during its conduction and the other of said tubes operable to provide a turn OFF signal during its conduction. a

9. The combination as set forth in claim 8 in which said tubes have an on-off ratio variable over a wide range and inwhich said tube conductive during the turn ON signal is conductive for a relatively shorter time duration than said tube conductive during the turn OFF signal.

10. The combination as set forth in claim 9 in which said transformer saturates during the turn OFF signal and prior to turn ON signal.

11. The combination as set forth in claim 8 in which said tubes have an on-ofi ratio variable over a wide range and in which said tube conductive during the turn ON signal is conductive for a relatively longer time dura: tion than said tube conductive during the turn OFF signal.

' 12. The combination asset forth in claim 11 in which a parallel resistor-capacitor-networkshunted by a diode phased to conduct drive current isconnected in series with said secondary to limit reverse current through said 7 V prevent saturation of said transformer during ON time.

13. A coupling transformer stage between a periodically pulsed vacuum tube amplifier and at least one power transistor having a load and a power source serially connected across its principal electrodes comprising a transformer having its primary connected tothe output of said amplifier and its secondary connected across the 7 control electrode and one of said principal electrodes of said transistor nonconductivefor a selected time duration subsequent to its being rendered conductive.

14. The combination as set forth in claim 13 in which said biasing means comprises a semiconductor diode and a parallel connected capacitor.

15. The combination as set forth in claim 14 in which a parallel resistor-capacitor network is connected in series with said primary for accelerating the changeof state of said transistor from .nonconductive to conductive.

16. The combination as set forth in claim 13 in which said transformer is a pulse transformer and an inductance is connected across said secondary for accelerating the change of state of said transistor from conductive to nonconductive.

17. The combination as set forth in claim 13 in which said amplifier'includes a pair of tubes connected to said primary in push-pull relationship and said transistor is driven conductive and nonconductive under power.

18. .The combination as set forth in clahn 14- in which conduct drive current is connected in series with said' secondary to limit reverse current through said transistor during its nonconductive period and to prevent saturation of said transformer during the conductive period of said transistor.

19. An electronic circuit for providing unidirectional power pulses of sharp turn ON and turn OFF characteristics across a load comprising, a multivibrator biased to provide a pulsating output, a vacuum tube amplifier operatively connected to said multivibrator for amplifying its pulsating output, at least one power transistor having a base, emitter and collector, said transistor having its emitter and collector connected across the load and a serially connected power supply, said transistor operable to receive opposite polarity turn ON and turn OFF signals across its emitter and base, a pulse transformer of the step down type having its primary connected in the output circuit of said amplifier and its secondaryconnected across subsequent to turn OFF.

20. The combination as set forth in claim 19 in which said direct current biasing means comprises at least one semiconductor rectifier connected in parallel with a capacitor.

21. The combination asset forth in claim 19 in which a parallel resistor-capacitor network is connected in series with said primary for accelerating turn ON of said transistor. V

22. An electronic circuit for providing unidirectional power pulses of sharp turn ON and turn OFF characteristics across a load comprising, a multivibrator biased to provide a pulsating out-put, an amplifier operatively connected to said multivibrator for amplifying its pulsating output, said amplifier including a first and a second electronic tube, a transformer having its primary connected to said tubes for push-pull operation, at least one power transistor having its principal electrodes connected across the load and a serially connected power source,

said transformer having its secondary connected; across current biasing means connected in series with said sec-' ondary for maintaining said transistor nonconductive for a selected time duration subsequent to turn OFF signal.

23. The combination as set forth in claim 22 in which said biasing means comprises at least one semiconductor rectifier and a' capacitor connected in parallel.

24. The combination as set forth in claim 22 in which a parallel resistor-capacitor network is connected in series with said primary and the principal electrodes of one'of said tubes for accelerating the turn ON of said transistor.

25. The combination as set forth in claim 22 in which said transistor is of the PNP type having said secondary connected across its base and emitter electrodes and the load connected across its emitter and collector in common emitter relationship.

26. An electrical discharge machining circuit for machining a conductive workpiece by intermittent electrical discharge from an electrode tool across a dielectric filled gap comprising, a pulse generator adjustable to provide a pulsating output over a broad range of duty factor, an amplifier. for increasing the strength of the pulsating output of said pulse generator, said amplifier.

including a pair of electronic tubes alternately conductive responsive to the pulsating output of said pulse generator, a transformer having its primary connectedto said tubes for push-pull operation, and at least one power transistor having its principal electrodes connected across a serially connected source of machining power and said gap, said transformer having its secondary connected across one of said principal electrodes and the control electrode of said transistor for providing alternate turn ON and turn OFF signals thereto.

27. The combination as set forth in claim 26 in which said tube conductive during the turn ON signal is conducitve for a relatively shorter time duration than said tube conductive during the turn OFF signal and said transformer saturates during the turn OFF signal.

28. The combination as set forth in claim 26 in which abiasing means self-developed by drive current is connected in series with said secondary for maintaining said transistor non-conductive for a selected time duration after turn OFF signal.

29. The combination as set forth in claim 28 in which said biasing means comprises at least one semiconductor rectifier connected in parallel with a capacitor.

3G. The combination as set forth in claim 29 in which said tube conductive during the turn ON signal is conductive for a relatively longer time duration than said tube conductive during the turn OFF signal and a parallel resistor-capacitor networn shunted by a diode phased to conduct drive current is connected in series with said secondary to limit reverse current through said transistor during OFF time and to prevent saturation of said transformer during ON time.

31. The combination as set forth in claim 26 in wh ch said transistor is embodied as a PNP transistor having said secondary connected across its base and emitter and having its emitter and collector connected across said gap.

32. The combination as set forth in claim 26 in which a parallel resistor-capacitor network is connected in series with said primary for accelerating turn ON of said transistor.

33. The combination as set forth in claim 26 in which said pulse generator comprises an astable multivibrator including a pair of electronic tubes biased for alternate operation and having their control grids connected through a common rheostat for selective adjustment of duty factor.

34. An electrical discharge machining circuit for machining a conductive workpiece by intermittent electrical discharge from an electrode tool across a dielectric filled gap comprising, a pulse generator adjustable to provide a pulsating output over a broad range of duty factor, an amplifier for increasing the strength of the pulsating output of said pulse generator, a pulse transformer having its primary operatively connected to the output of said amplifier, at least one power transistor having its principal electrodes connected across said gap and a serially connected source of machining power, said transformer having its secondary connected across one of said principal electrodes and the control electrode of said transistor for providing alternate turn ON and turn OFF signals thereto, and an inductance connected across said secondary for accelerating turn OFF of said transistor.

35. The combination as set forth in claim 34 in which a biasing means is connected in series with said secondary for maintaining said transistor nonconductive for a selected time duration after turn OFF signal.

36, The combination as set forth in claim 35 in which said biasing means is self developed by drive current and comprises at least one semiconductor rectifier connected in parallel With a capacitor.

37. The combination as set forth in claim 34 in which a parallel resistor-capacitor network is connected in series with said primary for accelerating turn ON of said transistor.

38. The combination as set forth in claim 34 in which said transistor is embodied as a PNP transistor having said secondary connected across its base and emitter and having its emitter and collector connected across said gap.

References Qited in the file of this patent UNITED STATES PATENTS 

1. A COUPLING CIRCUIT FOR CONNECTING DRIVE POWER FROM A PERIODICALLY CONDUCTIVE, VACUUM TUBE AMPLIFIER TO AT LEAST ONE POWER TRANSISTOR, HAVING A BASE, EMITTER AND COLLECTOR, SAID TRANSISTOR HAVING A LOAD AND A POWER SOURCE CONNECTED IN SERIES ACROSS ITS EMITTER AND COLLECTOR AND OPERABLE TO RECEIVE INPUT SIGNALS ACROSS ITS EMITTER AND BASE COMPRISING, A PULSE TRANSFORMER HAVING ITS PRIMARY OPERATIVELY CONNECTED TO THE OUTPUT OF SAID AMPLIFIER AND ITS SECONDARY CONNECTED ACROSS SAID BASE AND EMITTER OF SAID TRANSISTOR, AN INDUCTANCE CONNECTED ACROSS SAID SECONDARY FOR ACCELERATING TURN OFF OF SAID TRANSISTOR, AND A BIASING MEANS CONNECTED IN SERIES WITH SAID SECONDARY FOR MAINTAINING SAID TRANSISTOR NONCONDUCTIVE FOR A SELECTED TIME DURATION AFTER TURN OFF SIGNAL. 